The invention relates to fuel cell systems and methods having relatively long use lifetimes.
A fuel cell can convert chemical energy to electrical energy by promoting a chemical reaction between two gases.
One type of fuel cell includes a cathode flow field plate, an anode flow field plate, a membrane electrode assembly disposed between the cathode flow field plate and the anode flow field plate, and two gas diffusion layers disposed between the cathode flow field plate and the anode flow field plate. A fuel cell can also include one or more coolant flow field plates disposed adjacent the exterior of the anode flow field plate and/or the exterior of the cathode flow field plate.
Each flow field plate has an inlet region, an outlet region and open-faced channels connecting the inlet region to the outlet region and providing a way for distributing the gases to the membrane electrode assembly.
The membrane electrode assembly usually includes a solid electrolyte (e.g., a proton exchange membrane, commonly abbreviated as a PEM) between a first catalyst and a second catalyst. One gas diffusion layer is between the first catalyst and the anode flow field plate, and the other gas diffusion layer is between the second catalyst and the cathode flow field plate.
During operation of the fuel cell, one of the gases (the anode gas) enters the anode flow field plate at the inlet region of the anode flow field plate and flows through the channels of the anode flow field plate toward the outlet region of the anode flow field plate. The other gas (the cathode gas) enters the cathode flow field plate at the inlet region of the cathode flow field plate and flows through the channels of the cathode flow field plate toward the cathode flow field plate outlet region.
As the anode gas flows through the channels of the anode flow field plate, the anode gas passes through the anode gas diffusion layer and interacts with the anode catalyst. Similarly, as the cathode gas flows through the channels of the cathode flow field plate, the cathode gas passes through the cathode gas diffusion layer and interacts with the cathode catalyst.
The anode catalyst interacts with the anode gas to catalyze the conversion of the anode gas to reaction intermediates. The reaction intermediates include ions and electrons. The cathode catalyst interacts with the cathode gas and the reaction intermediates to catalyze the conversion of the cathode gas to the chemical product of the fuel cell reaction.
The chemical product of the fuel cell reaction flows through a gas diffusion layer to the channels of a flow field plate (e.g., the cathode flow field plate). The chemical product then flows along the channels of the flow field plate toward the outlet region of the flow field plate.
The electrolyte provides a barrier to the flow of the electrons and gases from one side of the membrane electrode assembly to the other side of the membrane electrode assembly. However, the electrolyte allows ionic reaction intermediates to flow from the anode side of the membrane electrode assembly to the cathode side of the membrane electrode assembly.
Therefore, the ionic reaction intermediates can flow from the anode side of the membrane electrode assembly to the cathode side of the membrane electrode assembly without exiting the fuel cell. In contrast, the electrons flow from the anode side of the membrane electrode assembly to the cathode side of the membrane electrode assembly by electrically connecting an external load between the anode flow field plate and the cathode flow field plate. The external load allows the electrons to flow from the anode side of the membrane electrode assembly, through the anode flow field plate, through the load and to the cathode flow field plate.
Electrons are formed at the anode side of the membrane electrode assembly, indicating that the anode gas undergoes oxidation during the fuel cell reaction. Electrons are consumed at the cathode side of the membrane electrode assembly, indicating that the cathode gas undergoes reduction during the fuel cell reaction.
For example, when hydrogen and oxygen are the gases used in a fuel cell, the hydrogen flows through the anode flow field plate and undergoes oxidation. The oxygen flows through the cathode flow field plate and undergoes reduction. The specific reactions that occur in the fuel cell are represented in equations 1-3.
H2xe2x86x922H++2exe2x88x92xe2x80x83xe2x80x83(1)
xc2xdO2+2H++2exe2x88x92xe2x86x92H2Oxe2x80x83xe2x80x83(2)
H2+xc2xdO2xe2x86x92H2Oxe2x80x83xe2x80x83(3)
As shown in equation 1, the hydrogen forms protons (H+) and electrons. The protons flow through the electrolyte to the cathode side of the membrane electrode assembly, and the electrons flow from the anode side of the membrane electrode assembly to the cathode side of the membrane electrode assembly through the external load. As shown in equation 2, the electrons and protons react with the oxygen to form water. Equation 3 shows the overall fuel cell reaction.
In addition to forming chemical products, the fuel cell reaction produces heat. One or more coolant flow field plates are typically used to conduct the heat away from the fuel cell and prevent it from overheating.
Each coolant flow field plate has an inlet region, an outlet region and channels that provide fluid communication between the coolant flow field plate inlet region and the coolant flow field plate outlet region. A coolant (e.g., liquid de-ionized water or other low conductivity fluids) at a relatively low temperature enters the coolant flow field plate at the inlet region, flows through the channels of the coolant flow field plate toward the outlet region of the coolant flow field plate, and exits the coolant flow field plate at the outlet region of the coolant flow field plate. As the coolant flows through the channels of the coolant flow field plate, the coolant absorbs heat formed in the fuel cell. When the coolant exits the coolant flow field plate, the heat absorbed by the coolant is removed from the fuel cell.
To increase the electrical energy available, a plurality of fuel cells can be arranged in series to form a fuel cell stack. In a fuel cell stack, one side of a flow field plate functions as the anode flow field plate for one fuel cell while the opposite side of the flow field plate functions as the cathode flow field plate in another fuel cell. This arrangement may be referred to as a bipolar plate. The stack may also include monopolar plates such as, for example, an anode coolant flow field plate having one side that serves as an anode flow field plate and another side that serves as a coolant flow field plate. As an example, the open-faced coolant channels of an anode coolant flow field plate and a cathode coolant flow field plate may be mated to form collective coolant channels to cool the adjacent flow field plates forming fuel cells.
The invention relates to fuel cell systems and methods having relatively long use lifetimes. In some embodiments, the relatively long use lifetimes can be achieved even when using one or more reactant gas streams that are unsaturated with water.
In one aspect, the invention generally features a fuel cell that includes a cathode flow field plate, an anode flow field plate and a proton exchange membrane. The proton exchange membrane includes a catalyst material and a proton exchange material. The catalyst material is incorporated in the proton exchange material. In some embodiments, the fuel cell can be in a system, such as a fuel cell stack, containing one or more additional fuel cells.
The catalyst material can be formed of a material capable of catalyzing the reaction of hydrogen and oxygen to form water. The catalyst material can be formed of a metal or an alloy, such as platinum or a platinum-containing alloy. The catalyst material can be in the form of discrete particles. The catalyst material can be present in an amount so that an electrical conductivity of the proton exchange membrane is about the same as an electrical conductivity of the proton exchange material. The proton exchange membrane can include from about 0.0001 milligrams to about 0.010 milligrams per square centimeter of the catalyst material per micrometer of proton exchange membrane thickness. The proton exchange material can be formed of a perfluorinated sulfonic acid material, such as NAFION(copyright).
In another aspect, the invention generally features a membrane electrode assembly that includes a cathode catalyst, an anode catalyst and a proton exchange membrane between the cathode and anode catalysts. The planar area of the cathode catalyst is from about 90% to about 99.9% of the planar area of the anode catalyst (e.g., from about 95% to about 99.5%, from about 97% to about 99%, about 98%).
The proton exchange membrane can be formed of a proton exchange material having a catalyst material incorporated therein.
In a further aspect, the invention generally features a fuel cell that includes a cathode flow field plate, an anode flow field plate, a proton exchange membrane between the cathode and anode flow field plates, a cathode catalyst between the proton exchange membrane and the cathode flow field plate, and an anode catalyst between the proton exchange membrane and the anode flow field plate. The region adjacent the inlet region of the cathode flow field plate and/or the region adjacent the inlet region of the anode flow field plate is substantially devoid of the cathode catalyst.
The proton exchange membrane can be formed of a proton exchange material having a catalyst material incorporated therein.
In another aspect, the invention generally features a proton exchange membrane that includes a porous reinforcing material impregnated with a catalyst material.
The proton exchange membrane can further include an additional catalyst material disposed within the porous reinforcing material of the other proton exchange material.
In a further aspect, the invention generally features a method of operating a fuel cell. The fuel cell includes a cathode flow field plate and an anode flow field plate. The method includes flowing a gas mixture containing a cathode gas through the cathode flow field plate, and flowing a gas mixture containing an anode gas through the anode flow field plate to produce a fuel cell power output greater than zero. The gas mixture flowing through the cathode flow field plate is not saturated with water and/or the gas mixture flowing through the anode flow field plate is not saturated with water. After flowing the gas mixtures for a period of time (e.g., at least about 20,000 hours, at least about 50,000 hours, at least about 100,000 hours), the fuel cell power output is at least about the same as the power output produced by the fuel cell when the method is performed with the first and second gas mixtures saturated with water.
In another aspect, the invention generally features a method of operating a fuel cell. The fuel cell includes a cathode flow field plate and an anode flow field plate. The method includes flowing a gas mixture containing a cathode gas through the cathode flow field plate, and flowing a gas mixture containing an anode gas through the anode flow field plate to produce a fuel cell power output greater than zero. The gas mixture flowing through the cathode flow field plate is not saturated with water and/or the gas mixture flowing through the anode flow field plate is not saturated with water. After flowing the gas mixtures for a period of time (e.g., at least about 20,000 hours, at least about 50,000 hours, at least about 100,000 hours), the fluorine concentration in the fuel cell product water output is less than or about the same as the fluorine concentration in the fuel cell product water output when the method is performed with the first and second gas mixtures saturated with water.